A proximity sensor is a device used to detect the presence of an object, for example a metallic object or ferrous object. There are a number of principals of operation used in proximity sensors, for example, variable reluctance, eddy current loss, saturated core, and Hall affect.
Saturated core sensors, for example, include a core made from a material that will magnetically saturate when exposed to a magnetic field of a certain flux density. As a magnetic object is moved towards the core assembly, a distance is reached where the magnetic field of the object finds the core to be the smallest reluctance path. As a result, the flux of the filed enters the core and, as the distance is decreased the flux density increases and eventually saturates the core. The saturation of the core causes the impedance of the coil to decrease. By measuring changes in the impedance of the coil, the presence for the magnetic object may be detected.
FIGS. 1 and 2 show an example prior art proximity sensor and FIG. 3 shows an example of a core used in the proximity sensor of FIGS. 1 and 2. FIGS. 1 to 3 will now be described.
The proximity sensor 100 includes a housing having a target-facing front sensor surface 104 and four side walls 106 extending at right angles from the surface 104. The side walls 106 and front sensor surface 104 form a hollow interior region in the housing 102. A sensing region 200 extends out from the target-facing front sensor surface 104 in a generally hemispherical shape.
In this prior art example, the housing 102 includes two flanges 108 extending outward from the opposite side walls 106. The flanges 108 have apertures 110 to permit the proximity sensor 100 to be mounted on a support member (not shown). The housing 102 may be made from a non-magnetic material having a fairly low electrical conductivity. An external connector 140 is provided to enable external connection to the internal electrical components.
A core 120 is located in the housing 102 and is used to generate the magnetic fields needed to sense target. The core 120 is preferably constructed from a highly permeable material such as soft iron, cast iron, transformer steel, or any other like materials. The material preferably has a relatively high permeability (for example in excess of 10,000). In the prior art proximity sensor shown in FIGS. 1 to 3, the core 120 comprises a substantially flat, rectangular member bent in four locations to form a shape having a head portion 220, two leg portions 222, and two foot portions 224. The head portion 220 is perpendicular to the two legs 222, and the two legs are perpendicular to the two feet 224. The two feet 224 are parallel to each other and coplanar extending oppositely from the respective legs.
The coil assemblies 118 are provided (one coil assembly per leg and foot) through which current is passed to create the magnetic field in combination with the core 120.